The use of settling velocity in defining the initiation of motion of heavy mineral grains, under unidirectional flow

The results of critical threshold experiments on four commonly occurring heavy minerals are described. The data are presented, in conjunction with comparable quartz data, using the non‐dimensional Shields’ Curve and the Movability Number (U*/ws). The results indicate that critical shear stress for material of high density is overestimated by use of Shields’ Curve, under smooth boundary conditions. Grain settling velocity is found to be a good indicator for the critical shear stress for grains of a wide density range. A physical explanation for the results is proposed.